The present invention relates generally to light frequency or wavelength control and more particularly to using particular forms optical gratings to detect, tune, and stabilize one or more light wavelengths concurrently.
Optical technology is progressing rapidly. Growing needs, particularly in the telecommunications industry, are driving this progress and there is currently a major impetus to improve existing optical systems and to develop new ones. Unfortunately, many desirable capabilities are still lacking and several major components are not completely meeting requirements or expectations. These failings have resulted in high costs in existing systems and are limiting the adoption of future systems. A brief discussion of conventional optical gratings is first presented here assist in understanding their limitations as they relate to the present invention.
FIGS. 1a-b (background art) depict two variations of traditional gratings. As can be seen, the shape of the groove can vary. FIG. 1a shows square steps and FIG. 1b shows blazed triangles, but other shapes are also possible, e.g., sinusoidal shaped grooves, and the physics is essentially the same.
Such xe2x80x9ctraditional gratingsxe2x80x9d were initially made of glass with grooves, and a few are still produced in this manner today. This, however, has a number of disadvantages. For instance, the density of the grooves is limited by the capability of the ruling engine, and the quality of the grooves produced tends to decrease as elements of the ruling engine wear from usage. Production of this type of gratings is time consuming and difficult, and the cost of such gratings is therefore high.
Molded and holographic gratings were invented later on, and their production cost is significantly lower than for glass gratings. Unfortunately, although suitable for many applications, these gratings tend to deteriorate in harsh environments. For example, in fiber optic communications, all optical components must operate for long periods of time in temperatures ranging from sub-zero to over eighty degrees Centigrade, and in humidity ranging from zero to 100 percent (see e.g., GR-468-CORE, Generic Reliability Assurance Requirements for Optoelectronic Devices Used In Telecommunications Equipment).
As can also be seen in FIGS. 1a-b, traditional gratings have the property that light has to shine on the grating surface from above. This limits the useful diffraction effect of such gratings to only one dimension, and multiple units need to be assembled if multiple dimensions (axes of direction) are required.
One example of an application where the need to work with multiple wavelengths and axes is common, and growing, is wavelength division multiplexing and de-multiplexing (collectively, WDM) in fiber optic communications. The use of traditional gratings in WDM usually requires either adhesives or mechanical fixtures to keep the assembly intact. Alignment is also needed to make sure that the gratings diffract light in the proper directions. The resulting assemblies formed with such traditional gratings thus tend to be significantly larger than the optical fibers being worked with and mechanical connectors are needed for connection. All of these considerations, and others, increase the cost in a fiber optic communications system.
A relatively recent invention is the fiber Bragg grating. The fiber Bragg grating is a periodic perturbation in the refractive index which runs lengthwise in the core of a fiber waveguide. Based on the grating period, a Bragg grating reflects light within a narrow spectral band and transmits all other wavelengths which are present but outside that band. This makes Bragg gratings useful for light signal redirection, and they are now being widely used in WDM.
The typical fiber Bragg grating today is a germanium-doped optical fiber that has been exposed to ultraviolet (UV) light under a phase shift mask or grating pattern. The unmasked doped sections undergo a permanent change to a slightly higher refractive index after such exposure, resulting in an interlayer or a grating having two alternating different refractive indices. This permits characteristic and useful partial reflection to then occur when a laser beam transmits through each interlayer. The reflected beam portions form a constructive interference pattern if the period of the exposed grating meets the condition:
2*xcex9*neff=xcex
where xcex9 is the grating spacing, neff is the effective index of refraction between the unchanged and the changed indices, and xcex is the laser light wavelength.
FIG. 2 (background art) shows the structure of a conventional fiber Bragg grating 1 according to the prior art. A grating region 2 includes an interlayer 3 having two periodically alternating different refractive indices. As a laser beam 4 passes through the interlayer 3 partial reflection occurs, in the characteristic manner described above, forming a reflected beam 5 and a passed beam 6. The reflected beam 5 thus produced will include a narrow range of wavelengths. For example, if the reflected beam 5 is that being worked with in an application, this separated narrow band of wavelengths may carry data which has been superimposed by modulation. The reflected beam 5 is stylistically shown in FIG. 2 as a plurality of parts with incidence angles purposely skewed to distinguish the reflected beam 5 from the laser beam 4. Since the reflected beam 5 is merely directed back in the direction of the original laser beam 4, additional structure is usually also needed to separate it for actual use.
Unfortunately, as already noted, conventional fiber Bragg gratings and the processes used to make them have a number of problems which it is desirable to overcome. For example, the fibers usually have to be exposed one-by-one, severely limiting mass-production. Specialized handling during manufacturing is generally necessary because the dosage of the UV exposure determines the quality of the grating produced. The orientation of the fiber is also critical, and best results are achieved when the fiber is oriented in exactly the same direction as the phase shift mask. The desired period of the Bragg grating will be deviated from if the fiber is not precisely aligned, and accomplishing this, in turn, introduces mechanical problems. Thus, merely the way that the fiber work piece is held during manufacturing may produce stresses that can cause birefringes to form in the fiber and reduce the efficiency of the end product grating.
Once in use, conventional fiber Bragg gratings may again require special handling. The thermal expansion coefficient of the base optical fiber is often significant enough that changing environmental conditions can cause the fiber to either expand or shrink to the extent that the period of the grating and its center wavelength shift.
From the preceding discussion of traditional and fiber Bragg gratings it can be appreciated that there is a need for optical gratings which are better suited to the growing range of grating applications. The parent patent applications to this one (as noted in the CROSS-REFERENCE TO RELATED APPLICATIONS section, above) introduce multidimensional optical gratings that are a considerable improvement over traditional and fiber Bragg type gratings.
In the ongoing effort to improve existing optical systems and to develop new ones, an are of particular need includes light frequency or wavelength control, i.e., detecting, tuning, and stabilizing light wavelengths. Indeed, members of the present inventors"" company have been active in this field and some examples here include: U.S. patent application Ser. No. 09/798,499, filed Mar. 1, 2001 for a Light Wavelength Meter, by Tsai; U.S. patent application Ser. No. 09/798,721, filed Mar. 1, 2001 for a Light Frequency Locker, by Tsai; U.S. patent application Ser. No. 09/967,090, filed Sep. 27, 2001 for ITU Frequency/Wavelength Reference by Chen; and U.S. patent application Ser. No. 09/967,436, filed Sep. 27, 2001 for Multi-Channel Wavelength Locker Using Gas Tuning by Chen.
The noted examples and other known art for light frequency or wavelength control, however, in general and relevant part, pre-date multidimensional optical gratings and do not recognize or apply the novel characteristics of these gratings to detecting, tuning, and stabilizing light wavelengths.
Accordingly, it is an object of the present invention to provide a system for detecting, tuning, or stabilizing light frequency or wavelength.
Briefly, one preferred embodiment of the present invention is a system for processing the frequency of a light beam produced by a light source. A grating block is provided that is able to receive the light beam and emit a diffracted beam having a position based on the frequency of the light beam. The grating block includes at least one planar or cubical type grating element. A detector is then able to receive the diffracted beam, and based on its position provide a measurement signal. A processor is then able to receive the measurement signal and determine the frequency of the light beam from it to detect the frequency or wavelength of the light beam.
Briefly, another preferred embodiment of the present invention is a method for processing the frequency of a light beam produced by a light source. A first step includes receiving the light beam into a grating block having at least one planar or cubical type grating element. A subsequent step then includes emitting a diffracted beam from the grating block at a position based on the frequency of the light beam. Another step then includes detecting the position of the diffracted beam and providing a measurement signal based there on. And another step then includes processing the measurement signal to determine the frequency of the light beam, thus detecting the frequency or wavelength of the light beam.
Briefly, still another preferred embodiment of the present invention is a system for processing the frequency of a light beam produced by a light source. A grating means is provided for receiving the light beam and emitting a diffracted beam having a position based on the frequency of the light beam. The grating means includes at least one planar or cubical type grating element. A detector means then receives the diffracted beam and based on its position provides a measurement signal. A processor receives the measurement signal and determines the frequency of the light beam from it to detect the frequency or wavelength of the light beam.
Some advantages of the present invention are that it does provide an accurate, economical, robust and compact system for detecting, tuning, or stabilizing light frequency or wavelength.
The invention is particularly accurate due to its use of diffractive characteristics in planar or cubical type grating elements.
The invention is also particularly economical. In one regard, this is due to the well-known nature of semiconductor-like fabrication techniques and materials that may be used. In another regard, this is due to the invention""s ability to employ multiple sets of optical characteristics concurrently, including non symmetrical or chirped characteristics. In this latter case, single embodiments the invention may serve to concurrently process multiple frequencies wherein discrete prior art systems would otherwise be required.
The invention may also be notably robust. Semiconductor-like fabrication materials are inherently robust. And when semiconductor-like fabrication techniques are used to integrate elements of the invention, potential points of weakness are removed.
The invention may also be highly compact, and it may impart this compactness to larger assemblies employing it. The invention""s multiple optical characteristic capabilities, and thus its multiple or frequency or frequency range handling capabilities are one factor here, permitting fewer instances of the invention to serve than might otherwise be required. The invention""s use of semiconductor-like fabrication is another factor, permitting construction of embodiments of the invention approaching and equaling the compactness of modern semiconductor devices.
These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of the best presently known mode of carrying out the invention and the industrial applicability of the preferred embodiment as described herein and as illustrated in the several figures of the drawings.